1. Field of the Invention
The present invention relates to optical communication equipment.
2. Description of the Related Art
Tunable lasers are used to generate optical carrier signals that can be modulated with data for transmission over fiber-optic networks. Optical components in such networks (e.g., optical switches) may include numerous tunable lasers. As a result of aging, output characteristics in a laser, such as an output wavelength at selected hardware settings, may change (drift) significantly over the laser's lifetime. To compensate for the drift, each laser is typically monitored and calibrated, preferably in a cost effective, non-disruptive, and dependable manner.
FIG. 1 shows a representative system 100 for transmitting data using an optical switch 102. Switch 102 is a 3×3 switch comprising a 3×3 arrayed waveguide grating (AWG) 104, three line cards 106 coupled to input ports of AWG 104, and three receivers 130 coupled to output ports of AWG 104. Each line card 106 comprises a tunable laser 110 and a modulator 120. Laser 110 feeds an optical carrier signal into modulator 120. Modulator 120 modulates the carrier signal with data to produce a data-modulated output signal of the respective line card 106. Each line card 106 can be configured to send its output signal to any chosen receiver 130 by setting the wavelength of laser 110 to the value corresponding to the desired output port of AWG 104.
System 100 further comprises one or more optical wavelength monitors (OWM) 112, typically one OWM 112 per line card 106. Each OWM 112 is configured to receive a small portion of laser output, analyze it, and generate a feedback signal. Using the feedback signal, laser 110 of the respective line card 106 can adjust its output to lock on a desired wavelength.
FIG. 2 illustrates one typical prior art implementation of OWM 112. OWM 112 comprises a plurality of optical wavelength lockers (OWL) 202, analog-to-digital (A/D) converters 204, a processor 206, and digital-to-analog (D/A) converters 208. The number of OWLs 202 in OWM 112 usually corresponds to the number of optical channels in switch 102. Different OWLs 202 are configured to different wavelengths corresponding to the respective optical channels in switch 102. The output of each OWL 202 is converted into a digital signal by A/D converters 204, processed by processor 206, and converted back into analog form by D/A converters 208 to produce a feedback signal applied to the corresponding laser 110.
OWLs are well known in the art and may be, for example, Santec OWL-10 or OWL-20 available from Santec Corporation of Japan. OWLs are fixed wavelength devices with a relatively narrow capture range of, e.g., 0.25 nm. Therefore, to monitor an N×N optical switch having N lasers, each of which can tune to N different wavelengths, one needs a total of N2 OWLs. With large optical switches, e.g., having 100 channels, such a system becomes large and expensive to implement.